Selection anduse of plug-insocket-outlettest devices

Best Prac�ce Guide 8 (Issue 2)

Electrical Safety First is indebted to the following organisationsfor their contribution and/or support to the development ofthis Guide, and in particular to the HSE for providing the initialdraft:

This is one of a series of Best Practice Guides producedby Electrical Safety First* in association with leadingindustry bodies for the benefit of electrical contractorsand installers, and their customers.

BSI Product Serviceswww.bsigroup.com

British Gaswww.britishgas.co.uk

Certsurewww.certsure.com

City & Guildswww.cityandguilds.com

GAMBICA Associationwww.gambica.org.uk

Electrical Contractors’ Associationwww.eca.co.uk

Health and Safety Executivewww.hse.gov.uk

Institution of Engineering andTechnologywww.theiet.org

NAPITwww.napit.org.uk

Olimatwww.olimat.co.uk

SELECT(Electrical Contractors’ Association ofScotland)www.select.org.uk

Published by:

Electrical Safety First

Unit 331

Metal Box Factory

30 Great Guildford Street

London SE1 0HS

Tel: 0203 463 5100

Email: bpg@electricalsafetyfirst.org.uk

Website: www.electricalsafetyfirst.org.uk

Electrical Safety First and other contributors believe that the guidanceand information contained in this Best Practice Guide is correct, but allparties must rely on their own skill and judgement when making use ofit. Neither Electrical Safety First nor any contributor assumes any liabilityto anyone for any loss or damage caused by any error or omission in thisGuide, whether such error or omission is the result of negligence or anyother cause. Where reference is made to legislation, it is not to beconsidered as legal advice. Any and all such liability is disclaimed.

© Electrical Safety Council. February 2015

In electronic format, this Guide is intended to be made availablefree of charge to all interested parties. Further copies may bedownloaded from the websites of some of the contributingorganisations.

The version of this Guide on the Electrical Safety First website(www.electricalsafetyfirst.org.uk) will always be the latest. Feedbackon any of the Best Practice Guides is always welcome – emailbpg@electricalsafetyfirst.org.uk

Electrical Safety First is supported by all sectors of the electricalindustry, approvals and research bodies, consumer interestorganisations, the electrical distribution industry, professionalinstitutes and institutions, regulatory bodies, trade and industryassociations and federations, trade unions, and local and centralgovernment.

*Electrical Safety First (formerly the National Inspection Council forElectrical Installation Contracting) is a charitable non-profit makingorganisation set up in 1956 to protect users of electricity against thehazards of unsafe and unsound electrical installations.

ELECTRICAL SOCKETTESTER

AimThe Health and Safety Executive (HSE) has expressedconcerns about instances where simple* socket-outlet testdevices have been relied on to demonstrate that socket-outlets are ‘safe’, either as part of the initial verification ofnewly-installed socket-outlets, or for the periodic testing ofexisting socket-outlets. This guidance is intended toaddress those concerns.

No socket-outlet test device (however sophisticated) canbe relied on alone to provide full assurance that a socket-

outlet is safe to use. This guidance therefore covers notonly simple socket-outlet test devices but also those ofmore advanced designs.

The guidance is intended to supplement the informationprovided with socket-outlet test devices, which mustalways be read and followed in order to ensure that thedevice is used safely and correctly.

The guidance is intended for skilled persons competent inelectrical testing only.

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Selection and useof plug-in socket-outlettest devices

*It is important to distinguish between simple socket-outlet test devices that have been available for many years andare in widespread use, and certain more advanced designs. Most simple socket-outlet test devices indicate basicwiring faults. There are other, more sophisticated, socket-outlet test devices available which will, in addition, displayeither the range of numerical values into which the earth fault loop impedance falls, or the numerical value of theloop impedance. Such devices are considered to be either ‘advanced’ or ‘professional’, rather than ‘simple’, socket-outlet test devices for the purposes of this Best Practice Guide.

IntroductionThere are a number of proprietary plug-in devices on themarket that are designed to give a quick and easyindication of the electrical condition of socket-outletcircuits.

Although all these devices will indicate some of the basicelectrical faults that may be found in socket-outlet circuits,the simpler versions cannot be relied on to indicate certainother faults, some of which can be dangerous.

Furthermore, no socket-outlet test device, including anadvanced or professional device, can alone provide fullassurance that a socket-outlet is safe to use. For example,none can detect an open ring final circuit, a looseelectrical connection, a case of unsatisfactory insulationresistance of circuit conductors, or a reversal of the neutraland protective conductors.

Similar advice applies if a socket-outlet test device is usedto test an extension lead.

A further fault that most types of socket-outlet test devicecannot detect is a reversal of the line and PEN conductorswithin the incoming electricity supply to the premises, ifthe installation forms part of a TN-C-S system. The reasonwhy this potentially dangerous fault cannot be detectedby the test device is that the voltages ‘seen’ by the devicebetween the various contacts of the socket-outlet areunaffected by a reversal in the polarity of the supply, asshown in Fig 1.

Fig 1.Reversal of polarity in the supply in a TN-C-S system

Socket-outlet test devices are commonly seen in tool bagsbut, unfortunately, due to a lack of knowledge of theirlimitations amongst some users, such devices are oftenused, inappropriately, as the sole means of checkingwhether a socket-outlet is safe to put into, or continue in,service.

This can only be checked by following the inspection andtesting procedures set out in Part 6 of BS 7671,Requirements for Electrical Installations (IET WiringRegulations).

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Types of socket-outlet test deviceThere are three types of socket-outlet test device: simple,advanced and professional. Examples are depicted below.

Simple devices are designed to detect various faults.However, they cannot indicate or make any measurementof the effectiveness of the protective earthing (the earthfault loop impedance), or identify some other dangerousfaults.

Advanced and professional devices are designed to detecta wider range of faults, and can indicate or measure theeffectiveness of the protective earthing.

These advanced and professional devices display eitherthe range of numerical values into which the earth faultloop impedance falls or the numerical value of the loopimpedance. This information must be interpreted by theuser of the socket-outlet test device to determine whetheror not the socket-outlet is adequately earthed for safety.

The interpretation process requires knowledge of themaximum value of earth fault loop impedance allowedfor the protective device that is relied on to provideautomatic disconnection in the event of an earth fault.The maximum value depends on the type and rating ofthe protective device.

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ELECTRICAL SOCKETTESTER

0.0 - 1.5

EARTH FAULT LOOPIMPEDANCE (OHMS)

1.5 - 2.5

2.5 - 5.0

5.0 - 10

10 - 50

50 - 100

Professional device

Simple device

Advanced device

Simple socket-outlet test devicesSimple socket-outlet test devices are usually similar in sizeand appearance to a 13 A plug and typically cost lessthan £20 each.

They are useful devices because they will generallyindicate whether a socket-outlet is functional and are ableto detect certain faults, including, in most cases, reversedlive and earth connections or the absence of an earth,which can be very dangerous faults.

However, although simple test devices are able to detectthe absence of an earth, they are unable to measure theearth fault loop impedance at the socket-outlet and mighttherefore imply that it is safe to use even where theearthing is dangerously defective.

This very important fact might not be stated on simpletest devices or the associated packaging or instructions,and is also not known by many users of such devices.

Simple socket-outlet test devices can therefore very easilymislead the user into believing a socket-outlet isacceptably safe when it is not.

If the test device does not display either thenumerical value of earth fault loop impedance or therange of numerical values into which the loopimpedance falls, then it is a simple socket-outlet testdevice and must not be relied upon to indicatewhether a socket-outlet is safe to use.

This is because the test device cannot verify that certaincritical safety requirements of BS 7671 are being met,including the adequacy of the protective earthing.

HSE investigations into simple socket-outlettest devices

The HSE tested a sample of simple socket-outlet testdevices after an HSE electrical inspector observed variousinstances where such devices were being usedinappropriately.

In two instances where simple socket-outlet test deviceshad been used to demonstrate to an inspector thatsocket-outlets in installations forming part of a TN-C-Ssystem were adequately earthed, the inspector found witha professional socket-outlet test device that the earth faultloop impedances at the socket-outlets were 22 Ω and218 Ω respectively.

Such impedances are far in excess of the maximumvalues providing automatic disconnection of supply bymeans of the circuit overcurrent protective device in theevent of an earth fault.

Tests undertaken by HSE showed that simple socket-outlettest devices would not indicate a problem unless the earthfault loop impedance exceeded very high values, whichcould be in excess of 20,000 Ω.

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Advanced socket-outlet test devicesAdvanced socket-outlet test devices typically cost between£50 and £100.

These devices are more complex than simple onesbecause they use additional components and technologyto determine and indicate earth fault loop impedance.The ease of using advanced devices is intended toencourage more frequent checking of this very importantparameter.

They have all the normal check functions of simple testdevices, including reversed line and earth or reversed lineand neutral.

An advanced device displays the range of numericalvalues into which the earth fault loop impedance falls.However, there can be cases where this information is notsufficiently precise to indicate whether a socket-outlet isadequately earthed for safety. This depends on theparticular range of values and the type and rating of theprotective device that is relied on to provide automaticdisconnection of supply in the event of an earth fault.

Professional socket-outlettest devicesProfessional socket-outlet test devices typically cost severalhundreds of pounds.

These devices meet the requirements of BS EN 61557-3.They usually take the form of an earth fault loopimpedance test instrument used in conjunction with alead equipped with plug to suit the socket-outlet that is tobe tested.

These devices display the numerical value of the earthfault loop impedance rather than a range of numericalvalues into which the measurement falls.

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Comparison of capabilities of the different socket-outlet test device typesTable 1 summarises the capabilities of the three different types of socket-outlet test device covered in this guide.

Table 1 – Capabilities of simple, advanced and professional socket-outlet test devices

Intended use Remarks

Indicate whether socket-outlet is functional ✓ ✓ ✓ Cannot, alone, indicate whether socket-outlet is safe to use

Detect reversed line and neutral ✓ ✓ ✓ Does not detect a line and neutral reversal within the supply network of a TN-C-S system (see Note 3)

Detect reversed line and earth ✓ ✓ ✓ Does not detect a line and neutral reversal within the supply network of a TN-C-S system (see Note 3)

Detect reversed neutral and earth ✘ ✘ ✘ Requires use of other test instruments(e.g. continuity test instrument)

Detect presence of earth ✓ ✓ ✓ Does not indicate whether socket-outlet is adequately earthed for safety. Simple socket-outlet test devices can easily mislead by showing ‘Earth OK’ (or similar indication) even when earth fault loop impedance is in excess of 20,000 Ohms

Display the range of numerical values into which ✘ ✓ ✘ Displayed range may not be sufficientlyearth fault loop impedance falls precise in some cases to indicate whether a

socket-outlet is adequately earthed for safety

Display numerical value of earth fault ✘ ✘ ✓ Value must be checked to establish thatloop impedance it is not greater than the maximum value for

the relevant protective device

May be used to measure values of earth fault ✘ ✘ ✓ Value must be checked to establish that loop impedance to certify or report on the it is not greater than the maximum value forinstallation the relevant protective device

Detect open ring final circuit ✘ ✘ ✘ Requires use of continuity test instrument

Detect loose electrical connection ✘ ✘ ✘ Requires inspection

Detect unsatisfactory insulation resistance ✘ ✘ ✘ Requires use of insulation resistance test instrument

Indicate whether RCD will operate effectively ✘ Note2 ✘ Requires use of RCD test instrument

Check whether voltage drop is within required ✘ ✘ ✘ Requires use of voltmeter and load, orlimits ohmmeter and calculation

Check for exposed live parts ✘ ✘ ✘ Requires inspection

Simple

Advanced

Professional

Notes.

1) ✓ Denotes device is capable. ✘ Denotes device is not capable.

2) Some types of advanced test device can carry out a functional test of an RCD protecting the socket-outlet.

3) Some socket-outlet test devices do have the ability to detect reversed line and neutral polarity in the supply network.Alternatively, this dangerous condition may be detected by use of an appropriate voltage detection device.

Training for users of socket-outlettest devicesUsers of socket-outlet test devices shouldbe trained to:

• use the socket-outlet test device correctly and safely

• know the capabilities and limitations of thedifferent types of socket-outlet test device

• understand the types of fault that can be present ata socket-outlet

• identify the means by which the electricalinstallation is earthed (for example, through anearth terminal provided by the electricity distributor(TN system) or through earth rods (TT system)

• know what earth fault loop impedance means, andwhat values of earth fault loop impedance areacceptable

• determine whether or not RCD protection is presentor should be provided

• know when to take further action.

Initial verification or testing aftermaintenanceNew, repositioned or replaced socket-outlets should notbe put into service until the required verificationprocedures have been completed and it has beenestablished that the requirements of BS 7671 have beenmet. (Regulation 134.2 refers.)

In particular, for new work, it is unsafe and thereforeunacceptable to energise a socket-outlet final circuit andthen to plug in a socket-outlet test device to check forbasic wiring faults.

Condition Reporting(Periodic Inspection Reporting)For condition reporting, inspection comprising carefulscrutiny and the appropriate tests of Chapter 61 ofBS 7671 should be performed.

The appropriate tests on a socket-outlet would normallyconsist of protective conductor continuity or ring finalcircuit continuity, insulation resistance, polarity, earth faultloop impedance and functionality, including the correctoperation of any RCD protecting the socket-outlet.

page 9©The Electrical Safety Council

SummarySocket-outlet test devices are useful because they willgenerally indicate whether a socket-outlet is functional.They are able to detect certain faults, including, in mostcases, reversed live and earth connections, which can bevery dangerous.

There are three types of socket-outlet test device: simple,advanced and professional.

Although all three types will indicate some of the basicelectrical faults that may be found in socket-outlet circuits,the simpler versions cannot make any measurement ofthe effectiveness of the protective earthing or indicatecertain other faults, some of which can be dangerous.

Simple socket-outlet test devices can also very easilymislead the user into believing that a socket-outlet isadequately earthed by showing ‘Earth OK’ or similarindication even when the earth fault loop impedanceis in excess of 20,000 Ohms.

If a socket-outlet test device cannot display the earth faultloop impedance, it should not be used to check whether asocket-outlet is adequately earthed for safety.

If a socket-outlet test device can display the range ofnumerical values into which the earth fault loopimpedance falls, it may still be necessary in someinstances to obtain the numeric value (using aprofessional socket-outlet test device) to confirm that asocket-outlet is adequately earthed for safety.

No socket-outlet test device (however sophisticated) canbe relied on alone to provide full assurance that a socket-outlet is safe to use.

The only means of checking whether a newly installedsocket-outlet is safe to put into service, or of determiningwhether an existing socket-outlet is safe to continue inservice as part of a formal electrical installation conditionreport (periodic inspection report) on the installation, is tofollow the inspection and testing procedures set out inPart 6 of BS 7671, using a full set of test instrumentscomplying with the relevant parts of BS EN 61557.

page 10 ©The Electrical Safety Council

Your Notes

page 11©The Electrical Safety Council

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